Use of copper as a wiring material and an electrode material has been conventionally studied, and copper is used in a conductive paste used for a printed circuit board, which is cured or dried at a relatively low temperature (200° C. or lower). When the conductive paste is used for an application, where the conductive paste is burned at a relatively high temperature (500° C. or higher), however, conductivity is reduced due to oxidation of the copper, then the conductive paste cannot be used in an oxidizing atmosphere, and can be only used in a special atmosphere, such as a nitrogen atmosphere, and a reducing atmosphere. From the reason as mentioned, silver is often used for an application where a conductive paste is burned in an oxidizing atmosphere of a relatively high temperature, but use of the silver has a problem that a cost is increased, as the cost of the silver is higher than that of the copper. Use of a silver-plated copper powder has been studied to attain characteristics in between copper and silver, and to reduce the cost. The oxidation resistance thereof is improved compared to copper, but use of the silver-plated copper powder still has a problem that it cannot be used for an application where a conductive paste is burned in an oxidizing atmosphere of a relatively high temperature.
In order to solve the aforementioned problems, for example, proposed is a multi-element alloy powder including silver, and at least two non-silver elements (see PTL 1). However, this proposal is intended to be used as an alloy powder for ceramic piezoelectric device. Therefore, the powder contains precious metals, such as palladium, and platinum, and has a high melting point. In addition, the powder is produced by an aerosol pyrolysis method. Therefore, the productivity thereof is low, and it is difficult to achieve a low cost.
Moreover, proposed is a lead-free brazing filler metal composed of an alloy including bismuth as a base (see PTL 2). However, the bismuth content is high, i.e., 80% by mass or greater, as well as the brazing filler metal is not intended to be applied for such use that it is burned at a relatively high temperature. Therefore, there is a problem that the conductivity is reduced, when the brazing filler metal is applied for use that it is burned in an oxidizing atmosphere of a relatively high temperature.
Furthermore, proposed are metal balls for a wiring board, which contain μg and/or Cu in an amount of 20% by mass to 80% by mass in total, and the remainder composed of Bi and unavoidable impurities, and are obtained by solidifying into balls having particle diameters of 0.03 mm to 0.5 mm, where a circularity of the metal ball is 5% or less of the particle diameters (see PTL 3). However, this proposal has a problem that these metal balls cannot be used as a filler of a conductive paste, because the particle sizes of the metal balls are large.
Accordingly, there are needs for a silver-bismuth powder having excellent solder wettability, low volume resistivity, and excellent conductivity for such use that it is burned in an oxidation atmosphere of a relatively high temperature (500° C. or higher), as well as a conductive paste, and a conductive film.